Pharmacological agents possessing the ability to block cellular transmembrane influx of calcium are capable of suppressing that portion of myocardial or vascular smooth muscle contractility which is dependent upon extracellular calcium. Church et al., Can. J. Physiol. Pharmacol., 58, 254 (1980); Fleckenstein, Calcium and the Heart, P. Harris and L. Opie, eds., Academic Press (1971); Nayler et al., Bas. Res. Cardiol., 76, 1 (1981); Calcium Blockers, S. Flaim and R. Zelis, eds., Urban and Schwartzenberg, (1982).
These pharmacological agents, termed calcium entry blockers, have been proven to be useful in the treatment of hypertension, cardiac arrhythmias, angina pectoris, and coronary artery vasospasm (a possible cause of sudden cardiac death syndrome). Circ. Res., 52, Suppl. I, (1983); Hypertension 5, Suppl. II, (1983). In theory, calcium entry blockers are thought to act by blocking calcium influx through discrete calcium channels (slow channels) in cell membranes. Various tissues exhibit relative differences in sensitivity toward the calcium blocking effect achieved by certain calcium antagonists, theoretically as a result of tissue specific differences in the calcium channels. Acta Pharmacol. Toxicol., 43, 5 (1978); loc. cit. 291 (1978); Microvascular Res., 5, 73 (1973); Am. Rev. Pharmacol. Toxicol., 17, 149 (1977). It is believed that the slow calcium current is responsible for activation of pacemaker cells in the sinoatrial node and the atrioventricular node of the heart. Verapamil, a known calcium channel blocking agent, is believed to slow conduction velocity through the atrio-ventricular node of the heart, in explanation of the mechanism of its anti-arrhythmic activity.